Buggy code:
public class CodeFormatterVisitor extends ASTVisitor { public static class MultiFieldDeclaration extends FieldDeclaration { FieldDeclaration [ ] declarations ; MultiFieldDeclaration ( FieldDeclaration [ ] declarations ) { } } public static boolean DEBUG = false ; private static final int NO_MODIFIERS = 0 ; private static final int [ ] SINGLETYPEREFERENCE_EXPECTEDTOKENS = new int [ ] { TerminalTokens . TokenNameIdentifier , TerminalTokens . TokenNameboolean , TerminalTokens . TokenNamebyte , TerminalTokens . TokenNamechar , TerminalTokens . TokenNamedouble , TerminalTokens . TokenNamefloat , TerminalTokens . TokenNameint , TerminalTokens . TokenNamelong , TerminalTokens . TokenNameshort , TerminalTokens . TokenNamevoid } ; private int chunkKind ; public int lastLocalDeclarationSourceStart ; private Scanner localScanner ; public DefaultCodeFormatterOptions preferences ; public Scribe scribe ; public CodeFormatterVisitor ( DefaultCodeFormatterOptions preferences , Map settings , int offset , int length , CodeSnippetParsingUtil codeSnippetParsingUtil ) { } public void acceptProblem ( IProblem problem ) { } private BinaryExpressionFragmentBuilder buildFragments ( BinaryExpression binaryExpression , BlockScope scope ) { } private CascadingMethodInvocationFragmentBuilder buildFragments ( MessageSend messageSend , BlockScope scope ) { } public boolean checkChunkStart ( int kind ) { } private boolean commentStartsBlock ( int start , int end ) { } private ASTNode [ ] computeMergedMemberDeclarations ( TypeDeclaration typeDeclaration ) { } private ASTNode [ ] computeMergedMemberDeclarations ( ASTNode [ ] nodes ) { } private boolean dumpBinaryExpression ( BinaryExpression binaryExpression , int operator , BlockScope scope ) { } private boolean dumpEqualityExpression ( BinaryExpression binaryExpression , int operator , BlockScope scope ) { } private final TextEdit failedToFormat ( ) { } private void format ( AbstractMethodDeclaration methodDeclaration , ClassScope scope , boolean isChunkStart , boolean isFirstClassBodyDeclaration ) { } private void formatBlock ( Block block , BlockScope scope , String block_brace_position , boolean insertSpaceBeforeOpeningBrace ) { } private void format ( FieldDeclaration fieldDeclaration , ASTVisitor visitor , MethodScope scope , boolean isChunkStart , boolean isFirstClassBodyDeclaration ) { } private void format ( TypeDeclaration memberTypeDeclaration , ClassScope scope , boolean isChunkStart , boolean isFirstClassBodyDeclaration ) { } private void format ( CodeFormatterVisitor . MultiFieldDeclaration multiFieldDeclaration , ASTVisitor visitor , MethodScope scope , boolean isChunkStart , boolean isFirstClassBodyDeclaration ) { } public TextEdit format ( String string , ASTNode [ ] nodes ) { } public TextEdit format ( String string , CompilationUnitDeclaration compilationUnitDeclaration ) { } public TextEdit format ( String string , ConstructorDeclaration constructorDeclaration ) { } public TextEdit format ( String string , Expression expression ) { this . scribe . reset ( ) ; long startTime = System . currentTimeMillis ( ) ; final char [ ] compilationUnitSource = string . toCharArray ( ) ; this . localScanner . setSource ( compilationUnitSource ) ; this . scribe . initializeScanner ( compilationUnitSource ) ; if ( expression == null ) { return null ; } this . lastLocalDeclarationSourceStart = - 1 ; try { <START_BUG> expression . traverse ( this , null ) ; <END_BUG> this . scribe . printComment ( ) ; } catch ( AbortFormatting e ) { return failedToFormat ( ) ; } if ( CodeFormatterVisitor . DEBUG ) { System . out . println ( ( "Formatting<seq2seq4repair_space>time:<seq2seq4repair_space>" + ( ( System . currentTimeMillis ( ) ) - startTime ) ) ) ; } return this . scribe . getRootEdit ( ) ; } private void format ( TypeDeclaration typeDeclaration ) { } private void formatAnonymousTypeDeclaration ( TypeDeclaration typeDeclaration ) { } private void formatCascadingMessageSends ( CascadingMethodInvocationFragmentBuilder builder , BlockScope scope ) { } private void formatClassBodyDeclarations ( ASTNode [ ] nodes ) { } private void formatEmptyStatement ( ) { } private void formatEmptyTypeDeclaration ( boolean isFirst ) { } private void formatGuardClauseBlock ( Block block , BlockScope scope ) { } private void formatLeftCurlyBrace ( final int line , final String bracePosition ) { } private void formatLocalDeclaration ( LocalDeclaration localDeclaration , BlockScope scope , boolean insertSpaceBeforeComma , boolean insertSpaceAfterComma ) { } private void formatMessageSend ( MessageSend messageSend , BlockScope scope , Alignment messageAlignment ) { } private void formatMethodArguments ( AbstractMethodDeclaration methodDeclaration , boolean spaceBeforeOpenParen , boolean spaceBetweenEmptyParameters , boolean spaceBeforeClosingParen , boolean spaceBeforeFirstParameter , boolean spaceBeforeComma , boolean spaceAfterComma , int methodDeclarationParametersAlignment ) { } private void formatNecessaryEmptyStatement ( ) { } private void formatOpeningBrace ( String bracePosition , boolean insertSpaceBeforeBrace ) { } private void formatStatements ( BlockScope scope , final Statement [ ] statements , boolean insertNewLineAfterLastStatement ) { } private void formatThrowsClause ( AbstractMethodDeclaration methodDeclaration , boolean spaceBeforeComma , boolean spaceAfterComma , int alignmentForThrowsClause ) { } private void formatTypeMembers ( TypeDeclaration typeDeclaration ) { } private void formatTypeOpeningBrace ( String bracePosition , boolean insertSpaceBeforeBrace , TypeDeclaration typeDeclaration ) { } private int getExtraDimension ( ) { } private boolean isComma ( ) { } private boolean isGuardClause ( Block block ) { } private boolean isMinus ( ) { } private boolean isMultipleLocalDeclaration ( LocalDeclaration localDeclaration ) { } private boolean isPartOfMultipleLocalDeclaration ( ) { } private boolean isSemiColon ( ) { } private boolean hasComments ( ) { } private void manageClosingParenthesizedExpression ( Expression expression , int numberOfParens ) { } private void manageOpeningParenthesizedExpression ( Expression expression , int numberOfParens ) { } public boolean visit ( AllocationExpression allocationExpression , BlockScope scope ) { } public boolean visit ( AND_AND_Expression and_and_Expression , BlockScope scope ) { } public boolean visit ( Argument argument , BlockScope scope ) { } public boolean visit ( ArrayAllocationExpression arrayAllocationExpression , BlockScope scope ) { } public boolean visit ( ArrayInitializer arrayInitializer , BlockScope scope ) { } public boolean visit ( ArrayQualifiedTypeReference arrayQualifiedTypeReference , BlockScope scope ) { } public boolean visit ( ArrayQualifiedTypeReference arrayQualifiedTypeReference , ClassScope scope ) { } public boolean visit ( ArrayReference arrayReference , BlockScope scope ) { } public boolean visit ( ArrayTypeReference arrayTypeReference , BlockScope scope ) { } public boolean visit ( ArrayTypeReference arrayTypeReference , ClassScope scope ) { } public boolean visit ( AssertStatement assertStatement , BlockScope scope ) { } public boolean visit ( Assignment assignment , BlockScope scope ) { }

Fix:
expression . traverse ( this , ( ( BlockScope ) ( null ) ) ) ;